Raman spectroscopy has been demonstrated as a powerful non-invasive analytical technology for material characterization and identification. However, for some composite materials, organic compounds, and biological samples, the strong fluorescence emission that is stimulated by the excitation laser often overwhelms the weak Raman signal.
Several techniques have been proposed to suppress the interference of the fluorescent emission. In one approach as disclosed by Fujiwara M, et al. in Applied Spectroscopy, Vol. 40, p. 137, 1986, the wavelength of the excitation laser is shifted to near-infrared (NIR) region. However, the Raman signal also becomes weaker, since the Raman scattering cross-section is inversely proportional to the fourth power of excitation wavelength. To compensate for the weak Raman signal, a higher laser power has to be used, which may damage the subject sample. Another approach uses deep UV laser for Raman excitation as disclosed by Bowman W D, et al. in Journal of Raman Spectroscopy, Vol. 9, p. 369, 1980. But the lasers at this wavelength are both bulky and expensive. In addition, the UV light may be harmful and invasive to certain samples.
Other approaches employ some laser modulation techniques. For example, by taking advantage of the fact that fluorescence emission and Raman emission have different decay times, the two spectra can be separated in the time domain by stimulating the material with an ultra short pulse laser as disclosed by Howard J, et al in Journal of Physics E: Scientific Instruments, Vol. 19, p. 934, 1986. This approach requires the pulse width of the laser to be in the order of pico-seconds. Commonly a nonlinear Kerr gate is used to separate the fluorescence emission from the Raman signal. Another approach, which is named as ‘shifted excitation Raman difference spectroscopy’ (SERDS), is proposed by Shreve ΔP, et al. in Applied Spectroscopy, Vol. 46, p. 707, 1992. In this approach, two similar Raman spectra with a small shift in wavelength are obtained using a tunable laser. The difference between the two spectra is used to reconstruct the Raman spectrum. This approach utilizes the fact that the fluorescence spectra are generally insensitive to the small shift in excitation wavelength, while the Raman peaks shift exactly in unison with the excitation wavelength. A simpler but less effective approach is proposed by S. E. J. Bell, et al. in Analyst, Vol. 8, p. 1729, 1998, which obtains the difference Raman spectrum by shifting the position of the spectrometer, thus avoiding the use of the tunable laser. Although the above disclosed techniques achieved some success, their instrumentation complexity and cost issues prevented their wide adoption.